1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device including a switching element of a current-source inverter and two diodes provided for preventing destruction of the switching element.
2. Description of the Background Art
Inverters have a function of converting direct current power into alternating current power, and a function of converting an amplitude, a frequency, or a phase of alternating current power. They are used in various applications such as motor control, a power supply unit, and a ballast for a discharge lamp. Inverters include a voltage-source inverter operating as a voltage source, and a current-source inverter operating as a current source.
Referring to FIG. 14, a current-source inverter 100 has an alternating current power source AC, switching elements SW1-SW4, and a load L. Switching elements SW1 and SW2 are connected in series, and switching elements SW3 and SW4 are connected in series. A set of switching elements SW1 and SW2 and a set of switching elements SW3 and SW4 are connected in parallel with respect to the alternating current power source. Load L is located to connect a point between switching elements SW1 and SW2 and a point between switching elements SW3 and SW4. In current-source inverter 100, alternating current power from alternating current power source AC is appropriately converted and applied to load L by controlling turning ON/OFF of each of switching elements SW1-SW4.
As switching elements SW1-SW4 of current-source inverter 100, a transistor such as an IGBT (Insulated Gate Bipolar Transistor), a bipolar transistor, or a field effect transistor may be used. When a reverse voltage (a voltage causing a collector potential to be higher than an emitter potential) is applied to a transistor, the transistor may be destroyed. Accordingly, to prevent the reverse voltage from being applied to switching elements SW1-SW4, a reverse blocking diode is connected in series with respect to a switching element. The reverse blocking diode has a cathode connected to a collector of the switching element, or an anode connected to an emitter of the switching element.
Further, when the reverse voltage is blocked by the reverse blocking diode, a reverse current (recovery current) may instantaneously flow into the switching element, and the switching element may be destroyed by a reverse voltage generated by the recovery current. Accordingly, to prevent the recovery current from flowing into switching elements SW1-SW4, a recovery current protection diode is connected in parallel with respect to a switching element. The recovery current protection diode has a cathode connected to a collector of the switching element, and an anode connected to an emitter of the switching element. Thereby, the recovery current flows toward the recovery current protection diode instead of flowing into the switching element.
Japanese Patent Laying-Open No. 62-210858 discloses a technique in which a pair of a reverse blocking diode and a circulation diode is packaged. Further, a circuit structure of a conventional inverter is disclosed in Akemi Sano et al., “Current-Source Inverter for Induction Heating Using IGBT”, Denko Giho, No. 28, 1994, pp. 54-59, and K. Nishida et al., “NOVEL CURRENT CONTROL SCHEME WITH DEADBEAT ALGORITHM AND ADAPTIVE LINE ENHANCER FOR THREE-PHASE CURRENT-SOURCE ACTIVE POWER FILTER”, IEEE Industry Applications Conference 36th Annual Meeting, 2001.
In current-source inverter 100, a reverse blocking diode and a recovery current protection diode are connected to each of switching elements SW1-SW4 as described above. Therefore, a module of current-source inverter 100 requires a space for disposing a semiconductor chip having a reverse blocking diode formed therein and a space for disposing a semiconductor chip having a recovery current protection diode formed therein, which has caused a problem that the module of current-source inverter 100 has a large size. Further, producing the module of current-source inverter 100 requires a step of producing a reverse blocking diode and a step of producing a recovery current protection diode, which has caused a problem that the number of production steps is increased.
These problems are not limited to the module of current-source inverter 100, and may arise in overall semiconductor devices having a structure that a reverse blocking diode and a recovery current protection diode are connected to a switching element.